The major aim of this project is to elucidate mechanisms controlling cell fate decisions in developing T cells. Precursor T cells undergo a testing process in the thymus to ensure that cells expressing useless or self-reactive receptors do not mature. These selection processes require T cell antigen receptor (TCR) signals, but the precise nature of these interactions determine whether the cells will live or die. Thymocytes also receive signals as they mature that direct them into specific lineages. Signals through the TCR impose a bias on lineage choice to specify the gamma-delta or alpha-beta T cell fate and again in the alpha-beta pathway, to specify the CD4 or CD8 T cell fate. In addition, signals through the TCR must integrate with evolutionarily conserved developmental signaling pathways, such as the Notch pathway, to guide precursors to their appropriate fate. In efforts to understand how Notch signaling is regulated in the thymus, we have successfully generated mutant mice for assessing Notch function in vivo that circumvent problems of redundancy in Notch receptors/ ligands, early lethality associated with deletion mutants, and those associated with ectopic and over expression. In one approach, we have manipulated the expression of Presenilins (PS1/2), the proteins required for generating the active form of Notch. Transgenic constructs, expressing a dominant negative form of Presenilin, produce a profound block in gamma-delta and alpha-beta T cell development while enhancing production of B and NK cells. Dominant negative Presenilin appears to mediate its effects through Notch since defects imposed by this transgene can be compensated by co-expression of another transgene encoding the actived form of Notch. In another approach, we have generated conditional null mutations of PS1/2, targeting gene deletion to specific stages of thymocyte development. In one of these, TCR expression is normal but the expression of CD5 (a negative regulator of TCR signaling) and CD4 maturation are attenuated. An activated form of Notch (the gain of function mutant) yields reciprocal results and expression of activated Notch rescues the developmental defects imposed by PS deletion. Since Notch regulates CD5 expression and CD5 regulates TCR signaling and the development of mature T cells, these results suggest that Notch may indirectly regulate CD4 maturation via TCR and CD5. In another conditional Presenilin deletion model, we observe all the phenotypic defects in hematopoietic development previously reported to be associated with Notch signaling mutations as well as new ones that we are currently investigating. In another strategy to manipulate Notch, we generated a truncated, dominant negative form of Numb (a natural antagonist of Notch) using the CD2 promoter to target expression to early stages of lymphocyte development. These mice have early blocks in T and B cell development (most notably at the pre-TCR/BCR signaling stages) and a severe reduction of mature B and T cells in the periphery. Unexpectedly, we find no phenotypic or functional evidence of Notch interference in this system. Rather it seems that at least one underlying problem arises at stages of proliferation where cells appear to receive receptor signals to initiate cell cycle, but die prematurely with slower kinetics and/or fewer rounds of proliferation. Overexpression of a full length form of Numb gives a reciprocal phenotype in that abnormally high numbers of T and B cells are generated. Collectively these studies indicate that both TCR and Notch influence cell fate decisions at several stages of thymocyte development and provide some hints of how these two signaling pathways may be associated.